Automate Kibana cloud settings page

[florent-leborgne]

That will hopefully also fix elastic/kibana#193431

See @theletterf's Gist here: https://gist.github.com/theletterf/56dbab3e7e328a9dd1c6a3a5e1f96544 that is a plan proposal to address the following:

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This issue's goal is to also include this page https://www.elastic.co/docs/reference/kibana/cloud/elastic-cloud-kibana-settings in what gets generated from Kibana's YAML setting sources.

It's not 1 page per feature like the rest of settings pages in that section, but rather an aggregate of all settings supported on ECH, to propose a dedicated view for ECH users and help them not having to go through 10+ pages individually.

Its particularity is that it has settings older than 9.x., as this page was moved there from the old "Cloud" unversioned docs before the migration to Docs V3. For this issue, we could:

• retroactively fit older settings listed on that page in individual yaml files

• check the list of what is in that page against what is specified as available in ECH in existing setting files (I think this page is more accurate than the old individual pages even though we should eventually check against the code)

• add a piece to the script, or a second script to generate this page too (we can be flexible about the structure I don't like its current structure)

We shouldn't have a side page that is not automated and only telling a partial (pre 9.x) story on its own in the middle of 9.x docs if we end up automating all of the actual individual settings pages.